Optic's

﻿

One of the most important phases of the study of colored stones concerns
the optical properties of the various gem species, which permit accurate,
safe and rapid identification of both mounted and un-mounted stones.
Prior to the recent introduction of optical gem-testing instruments,
testing procedures were limited mainly to destructive and very unreliable
tests that constantly exposed the jeweler to criticism and even lawsuits.
This is a major reason why the majority of jewelers have simply avoided
handling colored stones, other than perhaps a limited number of standard,
inexpensive stone-set rings. A knowledge of the properties of colored
stones and of identification procedures constitute fundamentals that
should be common knowledge to every jeweler.

A clear understanding of optical properties and their roles in identification
and grading begins with a study of the nature of light and its behavior
in the various gems. Optical properties, which represent the main source
of a gem's beauty, may be defined as those properties that determine
the effect a given substance has on light as it is transmitted by and/or
reflected from it. The following discussion of the nature of light includes
only the essential information that is needed in the study of gems and
in the application of this knowledge to identification, grading and
appraisal procedures.

THE NATURE OF LIGHT

Visible light, whether emitted by the sun or a candle or a light
bulb, is a form of RADIANT ENERGY. The portion of the sun's radiant
energy spectrum that we can see represents but a small fraction of the
total; the remainder is invisible. The ELECTROMAGNETIC SPECTRUM, which
includes infrared light, visible light, ultraviolet light, radio and
television waves, X rays, cosmic rays and all other forms of radiant
energy, is represented in Figure I. All of these have two properties
in common: (1) they travel in air at substantially the same rate of
speed (186,300 miles per second), and (2) they may be thought of as
traveling in a wave motion. Physically, these forms of wave motion differ
only in FREQUENCY (the number of vibrations, or cycles, per second)
and in WAVELENGTH (the linear distance between corresponding positions
on a wave). Figure 2 illustrates a wavelength. Although there is no
distinct dividing line between the various forms of radiant energy,
since they represent a continuous spectrum, there are extreme differences
in the wavelengths of the various portions of the spectrum, Radio waves
may be as much as six miles long, whereas, cosmic rays, at the other
end of the spectrum, are only on the order of one-trillionth of a centimeter
long! Since the velocity of all of this radiant energy in air may be
regarded as equal, and the wavelengths differ so widely, the frequency
of the short wavelengths have to be tremendous in comparison to that
of the long wavelengths. In other words, the longer the wavelength,
the lower the frequency; the shorter the wavelength, the higher the
frequency.

The portion of the electromagnetic spectrum that is of primary concern
to the gemologist is visible light. Visible light represents a spectrum
ranging from the longest wavelength (red) through orange, yellow, green
and blue, down to the shortest wavelength (Violet). When these wavelengths
are mixed together, as we ordinarily see them, the result is WHITE light.
The wavelengths of the visible spectrum range from approximately 7600
Angstrom units for red to approximately 4000 Angstrom units for violet.
An Angstrom unit (abbreviated A.U.) is one ten-millionth of a millimeter
(Figure 1). In order for violet light to travel as rapidly as red light,
its frequency must be approximately twice that of red light. This may
be likened to a man and a child walking for a distance of a mile side
by side. If the man's stride is four feet and the child's are two, the
child must take twice as many steps in order to maintain the same pace.

SOURCES OF LIGHT

There are two basic sources of light: INCANDESCENCE AND LUMINESCENCE.
A solid mass may be an incandescent source of visible light, as when
a metal is heated to a point where it begins to emit visible radiation.
Thus we say that a metal is "red hot" or is at a "white heat". An incandescent
light bulb contains as its principle element a tiny filament that is
heated by electricity to a white - hot temperature. In contrast, luminescence
does hot involve high temperatures.

Materials may be stimulated to emit visible light by a form of radiant
energy such as ultraviolent light, which is invisible; this is called
FLUORESCENCE. The typical fluorescent bulb used today for general illumination
operates by an electrical discharge that gives off ultraviolet radiation.
A fluorescent coating on the inside of the tube has the property of
transforming the energy of the ultraviolet into a visible wavelength.

A number of gemstones (particularly opal, kunzite and diamond) fluoresce
in various corers when subjected to ultraviolet radiation. If the reaction
continues after the stimulating radiation is removed, the resulting
light is called PHOSPHORESCENCE. Although there are other forms of luminescence,
fluorescence is the most important type.